Thermal magnetic circuit breakers having added means for providing arcing fault protection in residential load centers are currently available. Because of the low current conditions associated with arcing faults, most manufacturers recommend installing circuit breakers with arcing fault detection in each of the branch circuits associated within the residential to load centers. The use of such thermal magnetic circuit breakers equipped with arcing fault protection is further suggested to limit any so-called "nuisance tripping", which may occur upon implementation of power tools and the like, to the specific branch circuit in which the power tools are connected.
The use of circuit breakers having electronic trip units such as described in U.S. Pat. No. 4,672,501 entitled "Circuit Breaker and Protective Relay Unit" within residential load centers and lighting panelboards as described in U.S. Pat. No. 3,463,967 entitled "Panelboard Load Center", has heretofore been discouraged because of the required use of current transformers to sense the circuit current and provide the trip units with signals representative of circuit current. The large size and significant cost requirements of the current transformers results in a corresponding added cost and increased size in the circuit breakers which contain the transformers.
Thermal magnetic residential type circuit breakers, such as described in the U.S. Pat. No. 5,519,561 entitled "Circuit Breaker Using Bimetal of Thermal-magnetic Trip to Sense Current", are currently available for sensing and detection arcing faults within the branch circuits of residential load centers.
One approach to an economically feasible implementation of arcing fault protection in a residential load center and lighting panelboard enclosure is the use a thermal magnetic circuit breaker adapted for arcing fault detection as the main circuit breaker, per se. The low probability of the occurrence of an arcing fault to interrupt all the branch circuits could offset any inconvenience that may occur upon such interruption.
A deterrent to the use of a bimetal within a thermal magnetic trip unit for sensing arcing fault current within a main circuit breaker is the size requirement of the main circuit breaker trip unit bimetal for handling all the circuit currents. It is believed that the larger current rating required with the main trip unit bimetal would limit the sensitivity of the bimetal to the low currents associated with the arcing fault occurrence.
It would be economically feasible to use a single main circuit breaker having an electronic trip unit in a residential load center to provide overall circuit protection, including arcing fault protection, in series with corresponding branch circuit breakers having thermal magnetic trip units dedicated solely to overcurrent protection within the branch circuits. The implementation of the main circuit breaker to sense arcing fault occurrence within the branch circuit breakers would require a transducer sensitive to such arcing fault occurrence without causing nuisance tripping, as described above.
The recent advent of small-sized semiconductors for sensing circuit current may result in the replacement of current transformer in circuit protection apparatus utilizing electronic trip units for overcurrent determination.
U.S. patent application No. 6,094,330 entitled "Circuit Interrupter Having Improved Current Sensing Apparatus" describes a miniature semiconductor in the form of a giant magnet resistor, GMR, used in place of current transformers for sensing arcing current as well as long time, short time and instantaneous overcurrent conditions.
U.S. patent application No. 6,002,561 entitled "Arcing Fault Detection Module" describes a simple electronic circuit that includes an autocorrelation transceiver adapted for determining the presence of an arcing fault condition and for interrupting a protected circuit upon such an occurrence.
One purpose of this invention, accordingly, is to describe a residential circuit breaker load center utilizing thermal magnetic residential circuit breakers within each branch circuit for overcurrent protection while employing a main circuit breaker for providing both overcurrent and arcing fault protection to the branch circuits.